An internal combustion engine which performs a homogeneous charge compression ignition (HCCI) has been researched. A fuel (gasoline) and an intake air are previously well mixed together in an intake passage, and this air-fuel mixture is compressed in a combustion chamber to be auto-ignited. In this HCCI engine, a negative valve overlap (NVO) period in which both exhaust valve and intake valve are closed is established from a posterior time point of an exhaust stroke to an anterior time point of an intake stroke, whereby a combusted gas of the air-fuel mixture remains in the combustion chamber as an internal exhaust gas recirculation (EGR). During the NVO period, the internal EGR gas is compressed, so that the temperature in the combustion chamber is increased. A fuel is directly injected into the combustion chamber of high temperature. A part of the injected fuel is burned in the combustion chamber and the temperature in the combustion chamber is further increased, whereby the previously mixed air-fuel mixture is heated so that an ignitionability of the air-fuel mixture is improved. The other unburned injected fuel is closed in the combustion chamber with the internal EGR gas of high temperature, so that the unburned injected fuel is reformed to have high ignitionability. Thus, the ignitionability of the previously mixed air-fuel mixture is further improved. The auto-ignition combustion can be realized.
It should be noted that a precise control of an auto-ignition timing is required to perform the auto-ignition combustion efficiently. Thus, it is desirable to perform the auto-ignition combustion based on an actually detected auto-ignition timing. JP-2008-248831A shows an ion-current detecting method for a spark ignition (SI) engine. In this method, an auto-ignition combustion control is performed in a specified driving condition, and an ion-current flowing between a center electrode and a ground electrode of a spark plug is detected in order to indirectly detect the auto-ignition timing. It should be noted that an ion-current detector for an SI engine is well known. That is, a part of electric power supplied to a spark plug is accumulated in a capacitor, and this electric power is applied between a center electrode and a ground electrode of a spark plug. When the fuel is combusted to generate ion, the ion-current flowing between both electrodes is detected.
In order to detect the auto-ignition timing based on the ion-current, it is necessary to supply electric power to a spark plug so that the capacitor is charged prior to a detection of the auto-ignition timing. However, if electric power is supplied to the spark plug while the fuel exists in the combustion chamber, it is likely that the fuel is ignited by a discharged spark. In such a case, an engine torque may fluctuate or an exhaust characteristic may deteriorate due to a deviation of the auto-ignition timing.
In the method shown in JP-2008-248831A, during a NVO period, the spark plug receives electric power before the fuel is injected in order to avoid a deviation of the auto-ignition timing, whereby a fluctuation in torque and a deterioration in exhaust characteristic can be avoided.
However, according to the above method, although the capacitor can be charged without a deviation in auto-ignition timing, a detection accuracy of the ion-current may be possibly deteriorated. That is, after charging the capacitor, when the ion-current flows due to a fuel combustion during the NVO period, the voltage applied between both electrodes of the spark plug is decreased due to a voltage reduction of the capacitor, which may cause a deterioration in detecting accuracy of the ion-current. The detecting accuracy of a combustion condition in the combustion chamber may be also deteriorated.